This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
As with speakers, people emotionally defend their amplifier, so
choosing the best is difficult. However, some brands stand out as
being consistently good while others are consistently bad. Among the
good are HiFonics, Phoenix Gold, a/d/s/, and Precision Power.
Generally, "good" amplifiers tend to cost more (in money/watt) than
"bad" amplifiers. So when you see an amp advertising 300W for only
$100, and are comparing an amp with 50W for $300, you will usually find
that the 50W/$300 amp will be of much higher quality than the 300W/$100